(From company literature) Nikon PK-13 27.5mm Auto Extension Ring. Nikon extension rings can be used singly or in combination to vary the reproduction ratio of lenses from 50 to 300mm.

They are mounted between the camera body and the lens, As a rule, the effect becomes stronger the shorter the focal length of the lens in use, and the longer the focal length of the extension ring. Through the lens (TTL) exposure control automatically compensates for lens extension.